This application relates generally to polycarbonate sheet having improved fire resistance. Specifically, this application relates to polycarbonate sheet having fire resistance sufficient to pass certain government mandated tests for building materials.
Polycarbonate sheet is an advantageous material to use for various building applications because it can be made transparent, and it has impact resistance far superior to glass or transparent acrylic plastics. However, polycarbonate can be problematic from a fire resistance standpoint.
Various efforts have been made to improve the fire resistance of polycarbonate sheet. Specifically, many inventors have focused on producing polycarbonate sheet having improved fire resistance by adding various fire retarding chemicals to polycarbonate. Fire resistance enhancing additives for polycarbonate are described, for example, in U.S. Pat. No. 4,248,976. Others have focused on developing flame retardant coatings for polycarbonate, as described, for example, in U.S. Pat. No. 4,163,834. It is also known to improve the fire resistance of polycarbonate by incorporating a brominated polycarbonate copolymer as described, for example, in U.S. Pat. No. 5,612,163. Certain flame-retardant copolymers of polycarbonate have also been developed, as described in U.S. Pat. No. 5,175,198.
Unfortunately, each of these methods has presented certain drawbacks. Fire retarding chemical additives introduce added expense and often negatively influence the color stability, transparency and long term aging properties of polycarbonate sheet. Adding a brominated copolymer can also be problematic for the aforementioned reasons, and additionally, raises environmental issues. Adding flame retardant coatings to the sheet introduces additional expense and complexity to the manufacturing process. Finally, producing a copolymer will often greatly alter the physical properties of the finished sheet.
Commercial grades of polycarbonate sheet are typically manufactured from a heat stabilizer, a processing release agent, and a high viscosity branched polycarbonate resin and/or a high viscosity linear polycarbonate resin. Typical commercial grades of polycarbonate sheet are made from linear polycarbonate resin having a viscosity of 4.5 to 8.0 cm3/10 min (300 deg. xc2x0 C., 1.2 kg) (e.g, LEXAN(copyright) ML3403 and MAKROLON(copyright) 31XX), or branched polycarbonate resin having a viscosity of 4 to 6.5 cm3/10 min (300 deg. xc2x0 C., 2.16 kg) (e.g., LEXAN(copyright) ML3324 and MAKROLON(copyright) 1143). There are also some new branched sheet grades on the market (e.g., MAKROLON ku1-1243) that have and MVR of 8-11 cm3/10 min (300 deg. xc2x0 C., 2.16 kg).
There are many different government-mandated tests for evaluating the fire resistance of polycarbonate sheet building materials. One of the more aggressive tests is French norm NF P 92-505 (hereinafter xe2x80x9cthe French dripping testxe2x80x9d), which is incorporated by reference herein. In this test, a radiator is placed above a specimen of the test material supported on a grid. Cotton wool is placed in a receptacle below the test material. During the test, the radiator is turned on for 10 minutes, and droplets from the specimen may fall through the grid onto the cotton wool. If the cotton wool burns, the specimen is considered as failing the test. This test is described more fully below, and an apparatus for performing the test is shown in FIG. 1.
It has been discovered that the fire resistance properties of polycarbonate sheet may be greatly enhanced without resorting to any of the undesirable methods described above. Specifically, the fire resistance of polycarbonate sheet can be greatly enhanced by significantly reducing the viscosity of the polycarbonate resin in the sheet. Moreover, the fire resistance of the sheet can be further enhanced by carefully controlling the content of phosphorous-based stabilizer and processing release agent within specific ranges.
The fire resistant polycarbonate sheet according to the invention may comprise branched polycarbonate resin having an MVR above 11 cm3/10 minutes when measured at 300xc2x0 C. and 2.16 kg. Alternatively, the fire resistant polycarbonate sheet according to the invention may comprise linear polycarbonate resin having an MVR above 8 cm3/10 minutes when measured at 300xc2x0 C. and 1.2 kg. The fire resistant polycarbonate sheet according to the invention typically further comprises a phosphorous-based stabilizer and a processing release agent.
According to a preferred embodiment of the invention, the fire resistant sheet may be made from branched polycarbonate resin having an MVR above 13 cm3/10 minutes when measured at 300xc2x0 C. and 2.16 kg. In this embodiment, the sheet preferably further comprises from 0.003 to 0.007 total weight percent (i.e., based on the total weight of all the ingredients) of the phosphorous stabilizer, based on the weight of phosphorous in the phosphorous stabilizer. To clarify, this number is calculated by determining the weight attributed to the phosphorous atoms in the phosphorous stabilizer, and dividing by the total weight of all the ingredients.
According to a different preferred embodiment of the invention, the fire resistant sheet may be made from linear polycarbonate resin having an MVR above 10 cm3/10 minutes when measured at 300xc2x0 C. and 1.2 kg. In this embodiment, the sheet preferably further comprises from 0.002 to 0.01 total weight percent (i.e., based on the total weight of all the ingredients) of the phosphorous stabilizer, based on the weight of phosphorous in the phosphorous stabilizer.